This invention relates to the field of devices for coupling light from light sources into waveguides, and more particularly to beam converters and beam shapers.
Coupling and pumping efficiencies between illumination sources and optical waveguides, including solid state and fiber lasers, depend in part on how evenly the waveguides are filled by light from the illumination sources. Intermediate light beam transformations resolve beam quality in two orthogonal directions to achieve more even illumination of the waveguides.
Certain lasers emit beams originating from spot sources having shapes that depart from circularity. For example, diode lasers, on account of their planar geometry, typically emit beams originating from spot shapes that are elongated along one orthogonal axis with respect to the other. The short axis can have a height dimension that is almost diffraction limited, while the long axis can have a width dimension many times the diffraction limit. A convenient way to characterize the beam dimensions is by xe2x80x9cbeam quality factorxe2x80x9d, which is the product of the height or width dimension of the spot source times the numerical aperture of the spot source along the same axis. Beam quality factors in a ratio of 20 to 1 or higher are possible.
Waveguides, such as multimode fibers, generally contain circular transverse sections. Light coupled into the waveguides must be within area dimensions and numerical apertures of the circular waveguides"" entrances. Focusing optics image the light sources onto the waveguides"" entrances. However, the elongated spot source shapes of lasers, such as diode lasers, often do not fit within the entrance parameters of the waveguides or fill only a portion of the waveguides"" entrances, either of which reduces coupling efficiency.
Anamorphic and other complex focusing optics have been used to couple the light more efficiently by focusing the light differently along the two orthogonal axes. However, the width dimension of the elongated spot sources can be reduced only to the extent that the numerical aperture along the long axis of the spot sources can be effectively increased. The height dimension of the elongated spot sources can be increased only to the extent the numerical aperture along the short axis of the spot sources can be effectively reduced. Some losses are still apparent in focusing systems where even the best compromises in the two orthogonal directions do not fit within the area dimensions and numerical apertures of the waveguide entrances.
Although such geometric optical solutions assure that more rays from the illumination source reach the waveguide entrances within their numerical apertures, the uneven distribution of radiant energy in the two orthogonal directions remains unchanged. The cost of anamorphic and other complex focusing optics is quite high, and the diffraction images of the spot sources can be made worse by the generation of additional unwanted modes.
Regardless of their complexity, the focusing systems are incapable of increasing the radiance of the light distribution imaged onto the waveguide entrances. The concentration of light within a less elongated spot shape would involve an increase in brightness. The Radiance Theorem forbids imaging systems to increase in the number of photons per solid angle per effective area of the beam (assuming the object and image spaces have the same index of refraction).
A characteristic of elongated-shaped source beams, particularly those emitted by planar waveguide light sources, is a well-defined linear polarization. Typically, the electric field vector of such beams is oriented perpendicular to the long axes of the beams. My invention (among its embodiments) utilizes this polarization characteristic to effectively foreshorten the elongated dimension of the source beams. For example, two sections of an elongated beam can be divided into different orthogonal linear polarizations and effectively folded together along the beam""s long axis to increase brightness while reducing the apparent spot size of the source beam to one-half of its original size.
Both the increase in brightness and the improvement in the spot shape of the source beam can be accomplished at low cost and within a relatively small space. Circular or elliptical entrances of optical waveguides can be more evenly filled by the reshaped source beams for increasing coupling efficiencies between light sources and fiber transmission systems or pumping efficiencies between the light sources and optical amplifiers. The invention provides for depositing optical power from light sources into more compact regions of space and can be utilized in other situations where such brightness or beam shaping improvements are needed.
An exemplary beam converter arranged in accordance with my invention functions as an intra-beam polarization multiplexer. A linearly polarized beam having an initial transverse area propagates along an optical pathway within the converter. A polarization rotator interrupts a first transverse segment of the propagating beam for changing polarity of the first transverse segment of the beam with respect to a second transverse segment of the beam. A polarization-sensitive beam displacer combines at least portions of the first and second transverse segments of the beam within a common transverse area for enhancing brightness of the beam.
Preferably, the polarization rotator rotates polarity of the first transverse segment of the beam to a linear polarization that is orthogonal to the linear polarization of the second transverse segment of the beam. For example, the polarization rotator can be a retarder that delays a component of the first transverse segment of the beam for changing the polarity of the first transverse segment of the beam through 90 degrees with respect to the second transverse segment of the beam. A half-wave retardation plate having principal axes oriented at 45 degrees to the original polarization direction can be used to accomplish this.
Within the initial transverse area of the beam, long and short orthogonal axes are defined. The beam has a greater width along the long axis than along the short axis. The polarization rotator splits the long axis of the beam between the two transverse segments. The polarization-sensitive beam displacer laterally displaces one of the first and second transverse segments of the beam with respect to the other of the first and second transverse segments of the beam along the long axis so that the first and second transverse segments at least partially overlap. The displacement does not change the direction of propagation, so both of the original segments of the beam occupy a common region of space within which the radiance of the beam is increased.
Preferably, the polarization-sensitive beam displacer is a polarizer, such as a birefringent crystal. A calcite or similar crystal having a higher refractive index for polarization in a plane of incidence can be used to refract the two segments of the beam by different amounts according to their polarization. The result is a lateral shift of one beam segment with respect to the other. However, since the two beam segments are initially distinguished laterally, the lateral shift has the effect of overlapping the two beam segments. Dimensions of the crystal can be set to maximize the amount of overlap.
An exemplary system for optically coupling or optically pumping a waveguide with a source beam of linearly polarized light according to my invention includes a source that emits the beam of linearly polarized light from a spot source having area and numerical aperture dimensions along two orthogonal axes normal to a direction of propagation. The waveguide receives the beam of linearly polarized light through an entrance having area and numerical aperture dimensions along the two orthogonal axes that differ in at least one dimension from the corresponding dimensions of the spot source. A polarization rotator relatively rotates a polarization direction of a first transverse segment of the beam with respect to a polarization direction of a second transverse segment of the beam. A polarization-sensitive beam displacer relatively displaces the first and second transverse segments of the beam in a direction that modifies one of the area dimensions of the spot source. Imaging optics image the modified spot source onto the entrance of the waveguide within area and numerical aperture dimensions that would not be possible to achieve by similarly imaging the original spot source.
In a particularly useful application, the spot source has a width dimension along a first of the two orthogonal axes that is larger than a height dimension along a second of the two orthogonal axes, and a corresponding width dimension of the waveguide entrance along the first orthogonal axis is smaller than the width dimension of the spot source. The two polarization directions preferably correspond to the two orthogonal axes. The polarization-sensitive beam displacer relatively displaces the first and second transverse segments along the first orthogonal axis so that the modified spot source has a width dimension along the first orthogonal axis that is smaller than the width dimension of the original spot source. When imaged, the modified spot source better fits within the area and numerical aperture dimensions of the waveguide entrance.
Preferably, the polarization-sensitive beam displacer is a polarizer that is oriented with respect to two directions of polarization so that the two transverse segments are separately transmitted through the polarizer as ordinary and extraordinary beams. The polarization rotator is preferably a retarder that is oriented with respect to an initial polarization direction of the beam of linearly polarized light so that the polarization directions of the two transverse segments differ by 90 degrees. The retarder interrupts and divides the beam of linearly polarized light into the two transverse segments.
For purposes of increasing brightness, the polarization-sensitive beam displacer relatively displaces the first and second transverse segments in a direction that overlaps at least portions of the two transverse segments in a common direction of propagation. The area of the modified spot source is smaller than the area of the original spot source, while numerical apertures of the two spot sources are substantially the same. The increase in brightness does not violate the Radiance Theorem or produce interference because the overlapping beam segments have orthogonal linear polarizations.